Innate anti-predator behavior can promote infection in fish even in the absence of predators

Natural enemies - predators and parasites - largely shape the dynamics of ecosystems. It is known that anti-predator and anti-parasite defense can be mutually conflicting, however consequences of this trade-off for the regulation of infection burden in animals are still poorly understood. We hypothesize that even in the absence of cues from predators, innate anti-predator behavior (“ghost of predation past”) interferes with defense against parasites and can enhance the infection risk. As a case study, we explore interactions between a commercial species, the rainbow trout Oncorhynchus mykiss, and its parasite, the trematode eye-fluke Diplostomum pseudospathaceum. Fish-parasite interactions were tested in compartmentalized tanks where shelters and parasites were presented in different combinations providing various conditions for microhabitat choice and territorial behavior. Shelters were attractive and contestable despite the absence of predators and presence of parasites. The individua...